There is no direct method in the JavaScript Object Model to achieve what you are doing.
You will have to make another call to the 'TestList' to fetch the newly created item.

You can achieve this through the following way:

function Suceeded(){
//1) Query the 'TestList' to get items by Created Column.
//2) Set Ascending='FALSE' so that the latest created item will be fetched first.
//3) Set RowLimit of the Query to 1 so that only one item (the one you just created) will be fetched.
clientContext.executeQueryAsync(Function.createDelegate(this,this.FetchCurrentItemIdSucceeded),Function.createDelegate(this,this.Failed));
}
function FetchCurrentItemIdSucceeded(){
//Write the code here which was originally in the 'Suceeded' function.
}